1. Field of the Invention
The present invention relates to the tracking of phase angle in electrical conductors at remote locations. This includes not only identification of the phase of conductors, but may also include measurement of the actual phase angle.
2. Discussion of the Background
Electricity transmission and distribution systems normally employ multiple phases, typically three phases. Correct identification of the conductors at any given point in the system is very important. For example, small loads such as residential users are typically divided between the phases, and ideally should be divided equally, i.e. the loads should be balanced. Therefore, it is important to be able to choose a particular phase when connecting new consumers. Further need for phase identification arises when two sources of electricity have to be connected together. This is necessary for closing loops in overhead and underground distribution and for connecting multiple distribution feeds to a power network. Inadvertent connection of sources belonging to different phases can cause catastrophic damage to equipment and is highly dangerous to operating personnel. Although key points in transmission and distribution systems may be tagged with phase designations, over time the daily operation of the system tends to render such phase markings invalid. For example, a neighborhood that was once connected to one phase of a distribution system may have at some time been changed over to a different phase. Equipment upgrades may also give rise to the need to connect/reconnect to particular conductors carrying a particular phase.
Various methods are known for tracking or tracing the phase of electrical conductors in transmission and distribution. Visual tracing of overhead conductors is possible, but is time consuming and is prone to error. Such a technique is, of course, not possible where conductors are routed underground. To directly identify an underground conductor, it may even be necessary to de-energize the conductor of interest.
Gentile (U.S. Pat. No. 5,055,769) teaches determining phase relationships by synchronizing with an internal counter. However, in this system a single unit has to be physically carried from a main panel to a remote panel within a fine minute time limit, which limits the application of Gentile""s apparatus.
One known method of identifying the phase of conductors at a remote location is to send some form of a pilot signal on the conductor of interest. This is done, for example, in Allison et al (U.S. Pat. No. 5,617,329). In the system of Allison et al metering information from remote locations is transmitted in synchronism with a particular portion of the waveform, so that the phase of the load may be identified at a central location. No provision is made in Allison et al for this information to be sent back to the remote location.
Another known method is to send a sample of the waveform at a reference location to a remote field location for phase comparison at the field location. Systems of this type are shown in Mulavey et al (U.S. Pat. No. 3,027,523), Bouvrette (U.S. Pat. No. 4,626,622), Pometto (U.S. Pat. No. 5,510,700), and Hao (U.S. Pat. No. 6,130,531). Each of these systems has to be compensated for the time delay between the reference location and the remote field location.
Yet another approach to remote identification of the phase of conductors is used in Najam (U.S. Pat. No. 5,521,491). In the system of Najam the reference transmitter injects a data packet at each zero crossover, and a test receiver detects a received data packet and detects the local occurrence of zero crossover. However, the system of Najam does not appear to make any provision for propagation delay, as it is mainly aimed at use within a building. Also, the test apparatus includes only a receiver, and does not disclose any mechanism to initiate transmission of test signals from the reference transmitter, and therefore the reference transmitter must broadcast test signals on an essentially continuous basis.
Synchronized measurements of power waveforms making use of GPS and time tags have heretofore only been applied to fault location and/or the study of transient power disturbances (see Martin xe2x80x9cPrecise Timing in Electric Power Systems,xe2x80x9d 1993 IEEE International Frequency Control Symposium), and not to the uses described herein.
As power is delivered through an electrical transmission and distribution system, its power angle changes due to changes in the nature of the system, the generators attached, and the load attached. The power angle is the difference in phase angle between like voltage phases at two different points in the power system. Knowing the power angle of two different points in an electrical system tells an operator how power will flow once those points are connected together, and whether they can be connected safely. This information is useful for the operator of the electrical system, as catastrophic failure can occur should two points be connected where the power angles do not properly match. A need therefore also exists for comparison of power angle between different locations.
In one embodiment, the present invention employs a field unit and a reference unit both receiving a time signal from a common source, such as a satellite in the global positioning system (GPS), or a radio broadcast of a standard time signal, such as WWVB, and also provided with bi-directional wireless communication between the two units. As an alternative to a common time source, the field unit and reference unit may each be provided with an atomic clock, such as a Rubidium clock well known in the art, and the clocks may be synchronized to one another. It is of course possible for more than one field unit to be used with the same reference unit. The field unit is connected, either directly or via an external probe, to a conductor at a remote field location, and the reference unit is connected in a similar way to a reference conductor at a reference location.
In a common exemplary use, the field unit is connected to a conductor at a subscriber installation, while the reference unit is connected to a conductor at a substation. The field unit and the reference unit may in fact be connected anywhere in the distribution system, or anywhere in the high voltage transmission system with the aid of suitable probes.
In this embodiment, each of the reference unit and the field unit take readings of the phase of the main waveform at intervals determined by the time signal from the common source, and the field unit interrogates the reference unit by the bi-directional wireless link provided to obtain a comparison of the phase between the conductor at the field location and the conductor at the reference location. The result of this comparison is then displayed at the field unit.
The comparison of either phase angle or power angle need not be carried out in the reference unit, but may be carried out in the field unit. The comparative data may also be sent to equipment at a third location, such as a control center. This third location may be useful to independently verify and interpret the data presented on the field unit, so that a supervisor at the third location could help guide a field technician who is operating the field unit. It will also be appreciated that the bi-directional link between the field unit and the reference unit may not be a wireless link, but could be a fixed link, such as through the fixed telephone system or via the Internet.
In the above embodiments, measurements are taken simultaneously at the reference and field locations in synchronism with the time signal from the common source, and the measurement values are then associated with a xe2x80x98time tagxe2x80x99 so that measurements taken at each location can be compared for the same time period.